Optically pure α-amino acids are useful as a building block for designing various physiologically active substances and drugs. Recently, it was found that substances containing, in particular, a D-α-amino acid, which hardly occurs in nature, have unique physiological effects. Therefore, a process for conveniently obtaining an optically pure D-α-amino acid as a raw material is desired. Also, peptides and proteins composed of optically active unnatural synthetic α-amino acids have a more stable higher-order structure and an improved stability against hydrolytic enzymes than naturally occurring ones. Therefore, the importance of such optically active unnatural synthetic α-amino acids in drug development has been increasing, sad the development of a process for conveniently obtaining the optically active α-amino acids is an urgent issue.
As a production method of an optically active α-amino acid, optical resolution of a racemic mixture of an α-amino acid is classically known, and recently a fermentation method or an enzymatic method are known to easily produce L-α-amino acids. Regarding D-α-amino acids, deracemization of a racemic mixture and chirality inversion from an easily obtainable L-α-amino acid have been studied. Reported as examples of the methods are a method using a chiral ligand having an asymmetric carbon atom (see Non Patent Literature 1 etc.), a method using a chiral ligand having axial chirality (see Non Patent Literature 2, Patent Literature 1 and 2, etc.), etc.
However, in each method, there is a problem of generally slow inversion rate. In particular, in cases of amino acids having a sterically-bulky side chain, such as valine and isoleucine, there are problems of extremely slow reaction rate and low optical purity of the obtained product.
Consequently, none of the known methods are industrially satisfactory, and for the reason, the development of an industrially applicable production method of an optically active α-amino acid has been demanded.